Blender tapered stroke
Toggle the pressure settings in the paint tool bar for Radius and untoggle Strength so that you get a varied width but constant strength to your stroke. Enable Smooth Stroke either by toggle or pressing Shift+S, and possibly increase the samples to the stroke. This enables you to see the stroke lag behind the cursor and helps you round the stroke.

This is for freehand strokes - if you use the Curve Stroke tool and set up the path beforehand, there isn't a way yet to vary the width along the curve - it will evenly stroke along that curve.